The present invention relates to a combustible gas detection apparatus equipped with a combustible gas sensor to measure the concentration of a combustible gas component in a gas under measurement. The present invention also relates to a control method of a combustible gas sensor.
In recent years, intensive researches have been made on fuel cells as high-efficiency, clean energy sources in accordance with the social demands for environmental protection and nature conservation. Among others, polymer electrolyte fuel cells (PEFC) and hydrogen internal combustion engines are expected to be useful as energy sources for home and automotive applications because of their advantageous performance such as low temperature operability and high output density. As each of these fuel cell systems utilizes a combustible hydrogen gas as a fuel, the detection of a gas leak from the fuel cell system is one important problem to overcome. It is thus common practice to use, in the fuel cell system, a combustible gas detection apparatus for measuring the concentration of a combustible gas component in a gas under measurement.
There is known a conventional type of combustible gas detection apparatus, which includes a heating resistor placed in a gas under measurement and having a resistance that changes depending on a combustible gas concentration of the gas under measurement and a bridge circuit adapted to control the resistance of the heating resistor to a given level corresponding to a predetermined setting temperature and output a voltage across the heating resistor under such resistance control (referred to as “resistor control voltage”) as a detection value responsive to the combustible gas concentration.
In the above conventional type of combustible gas detection apparatus, however, the relationship of the gas concentration and the resistor control voltage varies with the resistance and heat capacity of the heating resistor during long-term use. This can result in an error in the gas concentration detection value (see FIG. 5A).
In view of such a problem, Japanese Laid-Open Patent Publication No. 2004-251862 discloses a combustible gas detection apparatus, which includes two adjacent heating resistors: one heating resistor is as a reference element and the other heating resistor is as a sensor element and diagnosis means for, at the time of deterioration diagnosis of the sensor element, simultaneously operating the reference element and the sensor element and comparing a gas concentration detection value of the reference element with a gas concentration detection value of the sensor element.